Precipitation of Salts upon Freezing

[liamkun]

Hi,

Im using the FREZCHEM.dat to look at salt precipitation on Freezing. However I cannot seem to look at the precipitation of multiple salts. In an Na-Cl-C(4)-H2O system, I set the equilibrium phases hydrohalite and ice(s) to 0 to look at precipitation but I get iteration errors when I run them together.

i can run hydrohalite and ice separately but not together and cannot work out why.

Also any idea why my solution concentration is different from the input in the first step (eg. 1.2M--> 1.4M [Cl]).

INPUT
   SOLUTION 1  G6
           units   mol/l
           pH      8.22
           pe      4
           density 1
           temp    25.0
           Na      2.4
           Cl      1.2
      C(4)   1.2
      water 1 # kg
   EQUILIBRIUM_PHASES
   Ice(s) 0.0 0
   REACTION_TEMPERATURE 1
      25 -45 in 70 steps
   END

All files attached.
- Ex1 -input-
- G6 -output-

[dlparkhurst]

First, concentrations differ from the input because of units conversion. Input is mol/L, whereas output is mol/kgw.

 The following simulation shows that you never achieve simultaneous ice and hydrohalite equilibrium at -30 C. The simulation removes water, allowing hydrohalite to precipitate. Regardless of the amount of water removed, the SI for Ice(s) reaches a minimum and then increases.

I can't say exactly why this happens. Perhaps the Pitzer parameterizations are not accurate under these conditions, or the equilibrium constant for hydrohalite is uncertain, or perhaps it is a physical reality. If you exclude C from the calculation or replace C with sulfate, you do find a point of equilibrium between Ice and Hydrohalite, but I am not sure what that means.

Code: [Select]

SOLUTION 1  G6
        units   mol/kgw
        pH      8.22
        pe      4
        density 1
        temp    25.0
        Na      2.4
        Cl      1.2
  C     1.2 charge
#        S(6)    0.6 charge
-water 1 # kg
EQUILIBRIUM_PHASES
Hydrohalite 0.0 0
REACTION_TEMPERATURE 1
-30
REACTION 1
H2O -1
50 in 100 steps
USER_GRAPH 1
    -headings               rxn SI_ice SI_hydrohalite m_Na
    -axis_titles            "Water removed" "SI" "Mol/kgw"
  -start
10 GRAPH_X RXN
20 GRAPH_Y SI("Ice(s)"), SI("Hydrohalite")
30 GRAPH_SY TOT("Na")
  -end
END


[MichaelZ20]

The solution with both Ice and Hydrohalite equilibrium may be received at

REACTION_TEMPERATURE 1
  -26  0  in  26 steps

but Hydrohalite at most the way does not precipitate.